Taxes on Japanese saving account

I have a saving account with Japan Post. I remember someone mentioning to me that in Japan you have to pay taxes once you exceed 1 million yen in your savings account. However, I can’t seem to find any information regarding this. Does anyone know if this is true and if so how much should I expect to pay in taxes on it? I’m about to hit that amount and if the taxes are high I might just transfer it to my home bank account if it costs less to keep it there.